The present invention relates generally to the assembly of semiconductor device packages onto printed circuit boards (PCB). More particularly, this invention relates to an improved stenciling apparatus and method for the deposition of solder paste material onto PCBs for the repair and rework of individual area array semiconductor device packages, also known as the ball grid array (BGA) and chip-scale package (CSP).
Circuit boards were initially developed using a plurality of holes for receiving leads from devices such as resistors, capacitors and transistors. The devices' leads were placed through the holes and soldered to the circuit board. The solder connected the lead to a conductive path on the circuit board. If a device did not work or was not properly connected to the circuit board, a user would use a soldering iron to heat the solder so that the solder could be removed by suction or other methods and the device removed. A new device then is placed on the board with the leads extending through the holes and then soldered in place.
As semiconductor devices/integrated circuits technology improved such that a greater number of elements and details could be placed in a smaller area, new technologies were developed such as surface mount technology (SMT) to allow for smaller and denser contacts or leads of semiconductor devices to be soldered directly to the surface of the printed circuit board (PCB). Area array packages, such as ball grid array (BGA) and chip-scale package (CSP), are becoming popular forms of high-density semiconductor packaging. Instead of leads extending their edges, these packages have an area array pattern of contact points or balls on the bottom side of the package that are not physically accessible after the device has been initially soldered to the PCB.
SMT utilizes a stencil printing method for applying solder paste to component pad patterns on the flat, planar surface of the unassembled PCB which already has the conductive paths on the board, using a metal stencil with apertures or openings corresponding to the component pad patterns on the PCB. The solder paste is a highly viscous material composed of fine solder particles blended with flux and other materials to render it printable. In this initial assembly process, sophisticated stencil printing machines align the frame-mounted stencil to the related PCB, bring the two into intimate contact, followed by a precision squeegee mechanism driving the solder paste material through the apertures onto the surface of the component mounting pads on the PCB in one pass. All component connection locations on the entire surface of the PCB are stenciled in one automated operation. The electronic components are subsequently placed onto the PCB with their contact points mating with PCB pad patterns onto which the solder paste has been precisely stenciled. The PCB with components thereon, then passes through an oven where the solder is melted or reflowed, thereby fusing the component contact points with the mating PCB pads.
Conventionally, individual semiconductor devices frequently require rework for the purpose of exchange, repair, or upgrading. As indicated above, BGA and CSP have their contact points on the bottom and are not accessible when mounted to the PCB. Rework of area array packages therefore requires removal of the device, and application of new solder paste to the PCB prior to reattachment of a replacement device. Due to the fact that the PCB is now populated with other components in close proximity, rendering the PCB a non-planar surface, the original primary stencil can no longer be used. This requires that new solder be re-applied to the component site by some other method.
One method to re-apply solder paste has been to utilize a syringe to dispense solder at each contact pad. However, this approach is slow and inconsistent in the amount of solder that is dispensed on each contact pad of a component footprint on the PCB (which can number hundreds of pads per component).
One of the more common techniques for re-application of solder paste to a single component site is to miniaturize existing primary SMT metal stencil technology. The mini metal stencil is a section of a primary stencil and uses the same technology. A flat mini metal stencil is custom sized to fit into the reworked space within the confines of surrounding components. The mini metal stencil may be formed with sides and features that allow for manual mechanical fixturing for alignment to the PCB pattern. However these mini metal stencils lack the mechanical support of the rigid stencil frame, as well as the automated functions of alignment, contact pressure, squeegeeing, and stencil lift-off that make the use of metal stencils successful in the primary SMT printing function.
The flat mini metal stencil is positioned by manual alignment and taping of the stencil's edges to the PCB. The formed mini metal stencil can be aligned with a mechanical arm, which requires set-up for each component location. Both of these methods can result in insufficient mechanical retention between stencil and PCB surface. Shifting of stencil position can occur during squeegee excursion or stencil removal requiring cleaning and re-stenciling. Alignment and positioning are difficult. Mini metal stencils must be cleaned thoroughly between each use.
Some of the difficulties with the mini metal stencil method include the possibility of not having enough solder paste deposited to form the contact pad or of having solder paste bleed under the stencil. In addition, the stencil might shift during squeegeeing passes. In that the stencil is made of metal, the stencil cannot be tailored with ease to fit the reworked site which is typically a confined space between other semiconductor devices. The result of these difficulties is the user of the mini metal stencils must acquire a high level of skill to successfully have the proper amount of solder paste deposited with only a single pass of the squeegee.